Various manufacturing methods for extruded synthetic resin foam have been well known so far. In general, methods have been adopted wherein a cell controlling agent is added to a synthetic resin material such as styrenic resin, melted and mixed together, and then the mixture is added to by a volatile-type foaming agent and extruded into a low pressure space to produce an extruded synthetic resin foam by the evaporation of the volatile-type foaming agent.
Among extruded synthetic resin foams manufactured according to the above-mentioned method, styrenic ones are favorable especially because they have a high expansion ratio and a uniform cell structure, and, in addition, exhibit excellent heat insulating properties and suitable mechanical strengths, and widely used usually as an insulating material and the like.
However, the extruded styrenic synthetic resin foam obtained with the above-mentioned method is basically polystyrene, which is high in rigidity. It is desirable to improve the flexibility of such a foam.
Thus, it is known to make an extruded synthetic (especially polystyrene) resin foam with fine and uniform cell structure to improve the foam's heat insulating properties and flexibility. However, making the cell size smaller involves a problem of increasing the density of the foam due to an effect of the surface tension of the cell membrane. Thus, it is difficult to obtain a synthetic resin foam having a low, not larger than 28 kg/m.sup.3 density with a uniform cell structure. It is desirable to make the foam body thicker by making the cell size smaller for the purpose of providing heat insulating properties and flexibility. Such intention, however, involves a problem that a resulting increase in the anisotropy of cell in the direction of the cell wall causes the heat insulating properties and the bending strength to reduce.
Actually in that connection, also the present inventors tried to make a styrenic resin foam having a fine and uniform cell structure by adjusting the amount of the foaming agent and the amount of the foam controlling agent. However, they failed in attaining the aimed heat insulating properties and flexural strength due to an increased anisotropy of resulting cells when the cell size was reduced to not more than 0.4 mm. Further, a reduction of the cell size to 0.2 to 0.3 mm made the foam body thinner rapidly, resulting in a difficulty of obtaining a synthetic resin foam having a satisfactory, thick body.
Furthermore, as a manufacturing method for an extruded synthetic resin foam, a method for manufacturing such a foam has been disclosed by U.S. Pat. No. 4,455,272. In this method, the melted resin in the extruding machine is pressure incorporated into by water to provide the resin with an expanding potential by water evaporation, and thus is cooled by the latent heat of evaporation.
Further, a method for obtaining a foam having a cell structure in which smaller and larger cells are formed together has been disclosed by U.S. Pat. No. 4,559,367. In this method, a water-containing organic vegetable substance having a 75 to 500 micron particle size is added in order to disperse the water uniformly.
In view of the above, the inventors eagerly sought to obtain a synthetic resin foam which possesses not only excellent heat insulating properties and suitable flexural strength which are essentially requested by any synthetic resin foam but also an excellent flexural strength. As a result, they found a system by which it is possible to manufacture an extruded synthetic resin foam which may satisfy all the above requirements and is constituted mainly by specific smaller cells and cells specifically larger in comparison with the former to complete the present invention.